Inherently flame resistant fibers are highly resistant to heat decomposition and are therefore desirable in the manufacture of flame resistant garments intended for environments in which flames or extreme heat will be encountered. These desirable properties of inherently flame resistant fibers can, however, create difficulties during fabric production. For example, fibers composed of aromatic polyamide, commonly known as aramid fibers, are difficult to dye. Aramid fiber suppliers have recommended complicated exhaust dyeing procedures with various dye-assistants, high temperatures, and long dyeing times to effect dyeing of these fibers. Such dyeing conditions require substantial amounts of energy both to maintain the dyeing temperature and for the treatment of waste dyebaths. Dye-assistants comprised of organic agents, and commonly referred to as carriers or swelling agents, are used to enhance dyeability. Such dye-assistants may be added to the dyebath as a treatment prior to dyeing, or can be integrated into the inherently flame resistant fiber during production.
Inherently flame resistant fibers such as aramid fibers can be blended with fibers made of other materials. As is known in the art, fiber blending can be used to obtain an end fabric that combines the beneficial characteristics of each of the constituent fibers. For instance, in the area of flame resistant fabric manufacture, flame resistant cellulosic fibers such as flame resistant rayon ("FR rayon") fibers can be successfully blended with aramid fibers to obtain a flame resistant material which is softer, more moisture absorbent, and less expensive to produce than materials constructed only of aramid fibers.
Although improving the texture and lowering the cost of flame resistant fabrics, blending inherently flame resistant fibers with flame resistant cellulosics such as FR rayon can complicate production. Specifically, cellulosics contain flame retardant agents that, although resistant to standard cellulosic dyeing procedures, tend to be depleted by the extreme temperatures generally considered necessary to dye the inherently flame resistant fibers. This depletion of flame retardant agents significantly reduces the flame resistance of the cellulosic fibers and therefore reduces the flame resistance of these blends. Moreover, these conditions increase the likelihood of further depletion of the flame retardant agents during subsequent launderings and an even greater reduction in flame resistance.
Due to the danger of depleting the flame retardant agent or agents contained in the cellulosic fibers of such fabric blends, producers of cellulosic fibers often advise their customers to avoid dyeing the inherently flame resistant fibers when blended with flame resistant cellulosic fibers. As an alternative, these producers suggest using producer colored inherently flame resistant fiber where a colored, flame resistant cellulosic blend is desired. In producer coloring (also known as "solution dyeing"), pigment or other coloring is typically injected into the polymer solution before the fiber is formed. Although providing for adequate colorization of these fibers, producer coloring presents several disadvantages. First, producer colored fibers usually are more expensive than non-producer colored fibers. Second, due to the increased difficulty and cost associated with the production of these fibers, typically only a limited variety of producer colored fibers are available.
Although dyeing at temperatures below 100.degree. C. substantially reduces the depletion of flame retardant agents from the cellulosic fibers, such low temperature dyeing creates a further complication. Specifically, when conventional dyeing methods are used at temperatures below 100.degree. C., not only do the inherently flame resistant fibers resist dyeing, these fibers become susceptible to substantial laundry shrinkage. Accordingly, if conventional piece-dyeing techniques are employed, the dyer is typically left with a choice between acceptable color and shrinkage control but unacceptable flame resistance on one hand (when dyeing above 100.degree. C.), and preserved flame resistance but high laundering shrinkage and poor color yield on the other (when dying below 100.degree. C.). Since neither option is commercially attractive, the industry preference for producer colored inherently flame resistant fibers in such blends is understandable.
From the above discussion, it can be appreciated that it would be desirable to have fabric blends comprising inherently flame resistant fibers and flame resistant cellulosic fibers in which the inherently flame resistant fibers have been dyed a full shade of color without depleting the flame retardant agent or agents contained in the cellulosic fiber, while simultaneously reducing the extent to which the fabric will shrink during laundering. Furthermore, it would be desirable to have a method for dyeing such fabric blends to achieve these properties.